A diverse number and type of inspection systems are available for inspecting samples for defects. Lasers are often used as light sources in many inspection systems to detect defects on wafers or photomasks. Lasers provide one of the most efficient illumination methods due to their extremely high brightness.
One of the downsides, however, of using lasers is that the high spatial and temporal coherency of laser light can cause a ringing effect when imaging patterns on the surface of a sample, or speckle when the surface features are random (e.g., due to surface or line edge roughness). The ringing effect or speckle can severely degrade image quality and introduce excessive noise, therefore reducing sensitivity for detecting defects. Comprehensive discussions about interference effects such as ringing and speckle phenomena can be found in “Fourier Optics”, by J. W. Goodman, McGraw-Hill, and “Statistical Optics”, also by J. W. Goodman, Wiley-Interscience.
These deleterious image effects can be reduced in various manners. One conventional technique of providing partially incoherent laser light involves the use of a rotating diffuser. A rotating diffuser typically consists of a rotating ground-glass screen that is introduced into the path of the laser beam before it reaches the object being imaged. The rotating diffuser introduces random phase variations into the incident laser beam, thereby reducing the spatial coherence of the beam. As the diffuser rotates, a detector can collect images of the object from independent views or perspectives. The detector, in turn, can integrate the independent inspection views to effectively synthesize an incoherent illumination of the object being imaged. One problem associated with using a rotating diffuser involves illumination efficiency. Diffusers are generally low efficiency because of excessive scattering of light. Another issue associated with a rotating diffuser is that it acts as a vibration source which is not ideal for an illumination system. Finally, the rotation speed of the diffuser disk must be faster than the integration time of the detector to provide adequate speckle suppression. This is not possible for high throughput systems where the integration time of the detector can be as low as a few nanoseconds.
In view of the foregoing, improved metrology apparatus and techniques for defect detection, while reducing speckle effects, are needed.